Hi CMS,
Thanks for contributing to our forum. I have seen the behavior you have described a few times before and, while I cannot necessarily predict when it may happen, you should see a warning indicating this has occurred if you click the orange number in the Notifications section along the top of any page within your Auctiva account.
As you suspected, it appears eBay's system determined your item belonged in a certain category based on the ISBN you provided and just automatically moved the item to that category (and sent back a notification indicating that was done) instead of rejecting the listing.
As far as I can tell, the only way to post such an item to a category other than the one eBay's system forces it into automatically is to remove the ISBN number from the Catalog Type section of the listing and then post it to eBay again.
I hope this helps. If you need any further assistance with your account, please feel free to contact our Customer Support team by mousing over the “Help” tab and selecting the “File Support Case” option.
-Mike